Some wafers may include defects, for example hairline cracks, other types of cracks, and voids, on a portion of the wafer, which are likely to make at least a semiconductor component formed on this part of the waver useless. The wafers may therefore be inspected in order to identify the defects, such that at least the respective semiconductor component can be discarded.
The identification of the defects may employ scanning acoustic microscopy (SAM, also referred to as acoustic micro-imaging (AMI)) using ultrasonic waves. It may represent a suitable method to analyze material properties or material change, and also to detect the defects, because it reacts strongly to interfaces between a solid material and a gas. It may be a non-destructive evaluation method commonly used in failure analysis.
FIG. 1 shows a SAM system 100 using ultrasonic waves 116 for detecting cracks 120 in a wafer 118. The system 100 serves as a sender 102 of the ultrasonic waves 116 emitted by a transducer 110 through a lens 112, and as a receiver 106 of ultrasonic waves reflected by the sample 118 entering the transducer 110 through the lens 112, with a switch 104 switching between sender and receiver. A frequency of the ultrasonic waves 116 may be in a range from about 5 MHz to about 500 MHz.
Ultrasonic waves can only be transmitted through solid state materials and liquids (there is no sound wave propagation in vacuum, for example). The ultrasonic waves may be transmitted from the transducer 110 to the sample 118 to be tested via a coupling medium 114. The coupling medium may for example be water 114, since there is almost no damping of the ultrasonic waves in water. As shown in FIG. 1, both the sample 118 and the transducer 110 may be placed in water 114, which means that the SAM-analysis may not be a contactless measurement technique.
A resolution of an acoustic image obtained by the SAM system 100 may depend on several factors, such as the frequency of the ultrasonic waves emitted by the transducer, focal length, numerical aperture, fluid path and signal strength.
FIG. 2 shows an impact of ultrasonic waves 116 on different defects 226 in a sample 118. Inside the sample 118, the ultrasonic waves 116 may be reflected, scattered, or absorbed.
Another conventional system includes an ultrasound system for detecting cracks in a sample.
FIG. 3 shows images 300 obtained by an SAM ultrasound system, for example by the system shown in FIG. 1 and FIG. 3, showing samples 118 with cracks 120. On the left side, a full view of the wafer is shown. All hairline cracks may be detected. Smallest details may be identified, as can be seen in the zoomed-in views of the cracks 120 shown on the right. Black points indicate particles on the wafer, caused by the measurement tool being installed outside the clean-room.
As can be seen in the bottom row of the images, also so-called star cracks 120, which may form in a middle of the wafer 118, may be identified.
However, an analysis using a SAM ultrasound system may not be suitable for a high production rate, because an average time required for analyzing one wafer 118 is about 45 minutes.
The analysis of the wafer 118 may be restricted to the edges only, which may save time, but then the star cracks 120 (in the bottom images in FIG. 4), which are not located at the edge of the wafer, will be missed.